The disclosure generally relates to epoxy adhesives and more particularly to a toughened epoxy adhesive with enhanced adhesion to sheet molding compound (SMC).
Sheet molding compound (SMC), for example, is defined (ASTM) as a molding compound in integral sheet form comprising a thermosetting resin, fibrous reinforcement, and additives required for processing or product performance, e.g., resin, catalyst, thickener, mold release agent, particulate filler, pigment, shrink control agent, etc. These materials and others generally are known as fiber-reinforced composites, reinforced composites, or simply composites. Metal may include, but not be limited to, hot dipped galvanized steel, electro galvanized steel, e-coat steel, cold rolled steel, bare aluminum, anodized aluminum, etched aluminum, magnesium, etc.
A common class of structural adhesives useful in adhering metal parts to the same and to different substrates (e.g. composites) is epoxy adhesives. Epoxy adhesive compositions most often contain a polyfunctional epoxy resin and are cured by addition of a curative, which typically is provided in a separate package, often referred to as Part A and Part B. The rate of cure and product characteristics are influenced by the choice of curing agent, which itself is influenced by the make-up of the adhesive composition, as dictated by the final properties desired by the user.
Acceleration of two-part epoxy adhesives has taken many paths over the years, such as, for example, the use of acrylic esters, mercaptans, and hydroxyl groups. The acrylic esters produce a rapid reaction, but typically result in brittle-glassy polymers with low adhesion to SMC. The mercaptans also react vigorously, but result in brittle polymers with low adhesion and have a strong odor. The catalytic affect of hydroxyl groups was first reported by Shechter, et al., Industrial Engineering and Chemistry, Vol. 48, No. 1, pp. 94-97 (1956) where a termolecular mechanism was proposed. Phenol was cited as being far more efficient than aliphatic alcohols due to its relative acidity. U.S. Pat. No. 4,129,670 claims the use of dihydric phenols, such as catechol or resorcinol, as epoxy chain extenders that “may reduce the cure time and/or temperature”. U.S. Pat. No. 5,385,990 claims the use of substituted aromatic phenols, where the substitution is an electron withdrawing group that causes an increase in the pKa value. This patent cites the previous work of U.S. Pat. No. 4,129,670 and, thus, excludes use of non-substituted aromatic phenols. Their preferred additive is p-chlorophenol, which is quite effective; however, it is plagued by an objectionable strong odor and recently chlorinated compounds, as a class, have been under scrutiny for being hazardous to the environment by automobile manufacturers.
U.S. Pat. No. 6,486,256 B1 cites the use of phenolic compounds as chain extenders, but claims they must be used with amines having only two hydrogen atoms per molecule, be free of polyfunctional curing agents, and must be used along with a base catalyst.
Additional representative epoxy structural adhesive compositions can be found in, for example, U.S. Pat. Nos. 5,385,990, 4,921,912, 4,661,539, 4,740,539, and 4,707,518, the disclosures of which are expressly incorporated herein by reference. Various combinations of epoxy resins, rubber modifiers, amine curing agents, amide curing agents, mercaptan curing agents, etc. have been proposed for formulating high strength adhesive compositions.